采用光谱展宽技术的钛宝石飞秒激光放大器

报道了飞秒激光啁啾脉冲放大器中的激光脉冲光谱展宽技术，实验中使用了几种光谱展宽元件，结果表明使用激光光谱展宽元件能有效地将飞秒激光脉冲 光谱展宽，抑制了飞秒激光放大器中常见的激光脉冲光谱增益窄化现象，从飞秒放大系统的振荡级输出28fs种子脉冲，放大后得到了重复频率为10Hz、峰值功率为1．6TW、脉冲宽度37．5fs的超短强飞秒激光脉冲 。     

镱钠共掺氟化钙锁模激光器产生190 fs光脉冲

在半导体激光器高功率抽运的镱、钠共掺氟化钙(Yb, Na:CaF₂)激光器中, 通过对晶体的负热透镜效应的数值模拟,进行激光器腔型的调节和优化,使得该激光器在采用2%的耦合输出镜和吸收抽运功率为7.8 W的条件下,获得了脉冲宽度为190 fs、平均输出功率为503 mW、中心波长为1034 nm、重复频率为82.4 MHz的连续锁模脉冲序列. 如果计算晶体表面反射等其他形式的泄漏输出,激光器的总平均输出功率为905 mW. 关键词： Yb 2晶体')" href="#">Na:CaF₂晶体 激光二极管抽运 飞秒激光     

两种基于交叉相位调制的光脉冲压缩方案

基于交叉相位调制的时间透镜可实现精确的二次相位调制, 但是它在光脉冲压缩领域的应用受到了抽运光脉冲的峰值功率过高的限制. 对该峰值功率的表达式进行了推导, 提出使用带有正色散的传输介质来实现输出段色散, 从而降低了抽运光脉冲的峰值功率.并进一步指出, 可以将基于交叉相位调制的时间透镜应用于4f系统, 来实现光脉冲压缩, 从而更有效地降低了抽运光脉冲的峰值功率. 推导了该系统的抽运光脉冲的峰值功率和分辨率的表达式, 并进行了光脉冲压缩的仿真分析.研究结果表明, 在基于交叉相位调制的4f系统中, 可以利用峰值功率较低的抽运光脉冲产生飞秒量级超短光脉冲; 随着压缩系数的提高, 输出光脉冲的脉冲宽度主要受到4f系统分辨率的限制, 并对4f系统分辨率的提高进行了讨论.     

Ultrafast femtosecond all-optical modulation through nondegenerate two-photon absorption in silicon-on-insulator waveguides

We present numerical investigations of ultrafast femtosecond (with time duration of 100 fs at 1/e intensity point) all-optical modulation of a pump-probe wave arrangement by using nondegenerate two-photon absorption (TPA), namely cross absorption, inside silicon-on-insulator (SOI) optical waveguides. Our results show that when a pump pulse with femtosecond duration and a continuous probe wave are co-propagating along the SOI, the probe wave can be modulated inversely by the ultrafast pump pulse, whose modulation depth depends strongly on the system parameters such as the waveguide length, the peak power and initial chirp of the pump wave, the group velocity dispersion (GVD), etc.; this means that the modulation depth can be improved by an appropriate increase of the waveguide length, the pump peak, and the initial chirp, in addition, which has a larger value for the probe wavelength in the normal dispersion regime compared with the case of abnormal dispersion when the center wavelength of the pump wave is located at the zero-dispersion wavelength.     

Apatite-structure crystal, Yb(3+):SrY(4)(SiO(4))(3)O, for the development of diode-pumped femtosecond lasers

We report what is believed to be the first diode-pumped Yb(3+): SrY(4)(SiO(4)) (3)O laser. We investigate both the cw and the mode-locked regimes. In the cw regime an average output power of 1.05 W is demonstrated for 3.6 W of incident pump power. In the femtosecond regime a pulse duration of 94 fs for an average power of 110 mW is obtained at a central wavelength of 1068 nm. This is, to our knowledge, the first femtosecond mode-locked oscillator made with an Yb-doped apatitelike crystal. These properties are related to the unique spectroscopic properties of Yb(3+) ions in this atypical apatite-family crystal.     

Two field nonlinear Faraday rotation in rubidium vapor in a Doppler-free geometry

Nonlinear Faraday rotation with counter-propagating light beams was investigated in a rubidium vapor cell. Observations across all hyperfine and crossover resonances of the D₂ line of ⁸⁷Rb atoms were made. Additionally, rotation spectra as a function of pump and probe intensity, pump polarization and external magnetic field strength were studied.The investigations were performed in a regime where the pump beam power was sufficiently low relative to the probe power to avoid polarization rotation due to pump-induced anisotropy. Our results are analyzed in the context of Bennett structures and coherent optical processes.     

Characteristics of Tellurite Neodymium-doped Glass Laser

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Highly stable, passively mode-locked fiber laser with low pump power for subpicosecond pulse generation

We present the design of highly stable, passively mode-locked fiber lasers which are based on a simple configuration and require a low pump power for the generation of optical pulses with subpicosecond pulsewidth. Our designed fiber laser uses a semiconductor saturable absorber mirror to realize the self-starting, passive mode locking at a low pump power of 29 mW, with which a stable optical pulse train is obtained at the fundamental repetition rate of 52.5 MHz and the central wavelength of 1557 nm. The output mode-locked optical pulses have a pulsewidth of 881 fs and an average output power of 1.78 mW, respectively.     

Laser pulse amplification upon Raman backscattering in plasma produced in dielectric capillaries

Femtosecond-pulse amplification upon stimulated Raman scattering is experimentally demonstrated for the case of a counter-propagating femtosecond laser pulse and a frequency-modulated broadband pump pulse with the same carrier frequency in a dielectric capillary filled with gas plasma. A value of ～10³ obtained for the spectral intensity amplification and ～10² for the output energy are the highest ever achieved for these quantities to date. Numerical simulation demonstrates good agreement with the experimental results. Based on the experimental data and the results of theoretical calculations, we propose a hydrodynamic mechanism for plasma-wave breaking as the mechanism playing an important role in the amplification restriction in the scheme considered.     

Observation of superradiant amplification of ultrashort laser pulses in a plasma

We demonstrate the amplification of a femtosecond signal pulse in an underdense plasma by a novel mechanism called superradiant amplification. The pulse is amplified by a counterpropagating few picosecond long pump pulse. In the superradiant regime, the ponderomotive forces exceed the electrostatic forces and arrange the plasma electrons to reflect the pump light into the signal pulse. We found a significant amplification in energy and intensity. The time structure of the amplified signal pulse carries intrinsic features of the superradiant regime. Sub-10-fs pulses of petawatt power appear feasible.     

Femtosecond pulse generation using stimulated Raman scattering in integrated silicon optical waveguide device

To generate ultrafast femtosecond optical pulses, we propose a model of an integrated device consisting of a Mach-Zehnder interferometer (MZI) with two symmetric 3 dB directional couplers and a straight waveguide based on the single-mode silicon-on-insulator (SOI) optical waveguide. The principle of pulse generation in the presented device is based on the strong stimulated Raman scattering (SRS) in silicon; the center wavelength of the pulse generated is tunable by changing the center wavelength of the co-propagating pump pulse. Numerical results show that, when a continuous wave (CW) with a weak power at 1670 nm wavelength and a pump pulse with a high peak power at 1550 nm wavelength are co-propagating, a narrow femtosecond pulse with a pulse width (full width at half maximum, FWHM) of ∼60 fs (FWHM of the pump pulse is 166.5 fs) can be achieved in the device proposed. In addition, when the waveguide length, pump peak power, and pump-pulse width are fixed, the properties of generated femtosecond pulse depend strongly on the incident chirp of the pump pulse and the CW power.     

Characteristics of Wavelength Tunable Femtosecond Soliton Pulse Generation Using Femtosecond Pump Laser and Polarization Maintaining Fiber

We have investigated both experimentally and numerically the characteristics of wavelength tunable femtosecond soliton pulse generation using a pulse width variable fiber laser and two different types of polarization maintaining fibers. The generated soliton shows the feature of the pulsewidth becoming almost constant at - 250 fs under any conditions of the pump pulse for 220m fiber. High conversion efficiency from pump pulse to a generated soliton pulse accounting for as much as 73% can be obtained. This efficiency decreases with increase in the input power or wavelength shift. A superior conversion efficiency and broad wavelength shift can be obtained by using a more shortened pump pulse. In the numerical calculations, it is predicted that under a condition of constant power of pump pulse, the maximum wavelength shift is achieved when the soliton number N is -1.4. The difference of mode field diameter and the group velocity dispersion (GVD) coefficient β₂ affect the wavelength shift and conversion efficiency. Using the fiber with small mode field diameter and small absolute value of GVD coefficient β₂, a high conversion efficiency and large wavelength shift can be obtained.     

Experimentally verified pulse formation model for high-power femtosecond VECSELs

Optically pumped vertical-external-cavity surface-emitting lasers (OP-VECSELs), passively modelocked with a semiconductor saturable absorber mirror (SESAM), have generated the highest average output power from any sub-picosecond semiconductor laser. Many applications, including frequency comb synthesis and coherent supercontinuum generation, require pulses in the sub-300-fs regime. A quantitative understanding of the pulse formation mechanism is required in order to reach this regime while maintaining stable, high-average-power performance. We present a numerical model with which we have obtained excellent quantitative agreement with two recent experiments in the femtosecond regime, and we have been able to correctly predict both the observed pulse duration and the output power for the first time. Our numerical model not only confirms the soliton-like pulse formation in the femtosecond regime, but also allows us to develop several clear guidelines to scale the performance toward shorter pulses and higher average output power. In particular, we show that a key VECSEL design parameter is a high gain saturation fluence. By optimizing this parameter, 200-fs pulses with an average output power of more than 1 W should be possible.     

光子晶体光纤飞秒激光非线性放大系统的耦合动力学过程研究

构建了掺镱大模场面积单偏振光子晶体光纤飞秒激光非线性放大系统. 讨论了腔内净色散量和抽运功率对振荡级输出参数的影响和振荡级参数对放大级输出参数的影响. 在本实验条件下, 当腔内净色散量取较大负色散时, 振荡级直接输出的脉冲更宽, 且携带更少的啁啾. 当振荡级抽运4.53 W时, 选择最接近变换极限的脉冲作为种子脉冲, 放大级在60 W抽运时输出压缩后无基底的短脉冲, 宽度为45.7 fs, 平均功率28 W. 振荡级抽运功率增加到5.08 W, 放大级抽运70 W时, 获得最高输出功率34.5 W, 对应脉宽53.5 fs.     

Dual picosecond dye lasers synchronously pumped by a mode locked cw yag laser

The performance of a novel dual dye laser system synchronously pumped by the frequency doubled output of a mode-locked CW-YAG laser is evaluated in relation to pulsewidth, pulse substructure, pulse spectral width and timing jitter. The behavior of the system is adequately described by a theoretical model which includes the time dependent gain and losses due to frequency bandwidth, cavity length mismatch and output coupler. The jitter is significantly reduced from that obtained with CW gas laser pumping as a result of the shorter pump pulse (50 ns instead of ≈100 ps). A routine operating condition uses 2-plate birefringen filters, 0.8 W pump power at 532 nm, to yield two 2.0 ps pulses having a cross correlation width of 3.8 ps, and 30 mW average power from each laser.     

飞秒时间分辨实验中相关函数和时间零点的确定

结合飞秒激光在研究分子激发态弛豫动力学中的应用,介绍了几种飞秒时间分辨实验中确定泵浦激光脉冲与探测激光脉冲的相关函数和时间零点的方法.对于波长在可见波段的泵浦和探测激光脉冲,我们可以利用非线性光学的技术手段,探测泵浦光与探测光的和频光的强度随二者间的时间延迟的变化来确定相关函数和时间零点;对于波长在紫外甚至更短的波段的泵浦和探测激光脉冲,由于单脉冲能量比较低,目前还很难利用技术手段来测定泵浦激光与探测激光的相关函数及时间零点,可以利用某些原子气体(如Xe)或某些具有短寿命态的分子作平行实验进行间接测量.     

Ultra-fast x-ray-dynamic experimental subsystem

Ultra-fast x-ray-dynamic experimental subsystem is a facility which can provide femtosecond hard x-ray sources using a femtosecond laser interacting with plasmas. By utilizing these ultra-fast x-rays as a probe, combined with a naturally synchronized driver laser as a pump, we can perform dynamic studies on samples with a femtosecond time resolution.This subsystem with a four-dimensional ultra-high spatiotemporal resolution is a powerful tool for studies of the process of photosynthesis, Auger electron effects, lattice vibrations, etc. Compared with conventional x-ray sources based on accelerators, this table-top laser-driven x-ray source has significant advantages in terms of the source size, pulse duration,brightness, flexibility, and economy. It is an effective supplement to the synchrotron light source in the ultrafast detection regime.     

High-Resolution Rb Two-Photon Transition Spectroscopy by a Femtosecond Frequency Comb via Pulses Control

We experimentally observe the high resolution direct frequency comb spectroscopy using counter-propagating broadband femtosecond pulses on two-photon transitions in room-temperature ~(87)Rb atoms.The Doppler broadened background is effectively eliminated with the pulse shaping method and the spectrum modulation technique.The combination of the pulse shaping method and the spectra modulation technique provides a potential approach to reduce background of at least 99%.     

Femtosecond high-power spontaneous mode-locked operation in vertical-external cavity surface-emitting laser with gigahertz oscillation

We realize a femtosecond high-power spontaneous mode-locked operation with gigahertz oscillation in a vertical-external cavity surface-emitting laser under the condition of eliminating the internal and external unwanted reflection. We find that the reflectivity of the output coupler has a significant influence not only on the output power but also on the output pulse duration. With an incident pump power of 20 W, we have achieved 2.35 W of average output power with 778 fs pulse duration at a repetition rate of 2.17 GHz. The shortest pulse duration was 654 fs at an average output power of 0.45 W.     

Measurement of laser-induced alignment of molecules by cross defocusing

The field-free alignment of CO2 produced in response to the excitation of a molecule by a high-intensity femtosecond pump pulse is measured with a simple coronography-like technique. The technique is based on the defocusing of a time-delayed probe pulse produced by the spatial distribution of aligned molecules. In the intensity regime explored here, the technique is shown to give valuable information about dynamic alignment. With the help of simulations, the degree of alignment is extracted from the data.